Universal testing machines (material testing machines) are widely used to measure mechanical properties of materials and components. The universal testing machine applies a stress, such as a tensile stress, a compressive stress or a bending stress, to a test piece, and measures the mechanical property, such as a breaking strength, an elastic coefficient and hardness based on behavior of the test piece. A typical universal testing machine includes a moving plate and a fixed plate, and applies load to the test piece by fixing one end of the test piece to the moving plate and fixing the other end of the test piece to the fixed plate (or by sandwiching the test piece between the moving plate and the fixed plate) and then moving the moving plate with respect to the fixed plate.
As an example of a mechanism for driving the moving plate of the universal testing machine, Japanese Patent Provisional Publication No. 2001-159593A describes a hydraulic mechanism. The hydraulic system mechanism controls a flow amount of operating oil to be supplied to a hydraulic cylinder by adjusting a discharge amount of a hydraulic pump driven by a servo motor, not by adjustment of opening of a servo valve. In the hydraulic mechanism, the flow amount of the operating oil to be supplied to the hydraulic cylinder is precisely controlled by controlling the number of revolutions of the servo motor which drives the hydraulic pump.